But what happens if you lose a leg?
The harness (aka the chafer) is probably the safest option, if awkward and not terribly comfortable. It is not uncommon for avalanche victims to have articles of clothing torn off, including pants, and backpacks can be especially vulnerable to getting caught and yanked off.
Pants pocket would be preferable to leaving it in the car, to be sure.
When I wear mine (so far, exactly once this season :/ ), I wear the harness. I think my next choice would be to put it in a pocket of a clothing layer, like a vest, next choice interior pocket of a jacket, exterior pocket, pants pocket. Last choice while still having the beacon with me would be backpack.
Amazing how much of this stuff there is to nitpick when it comes to avalanche safety, eh?